the nerve impulse

Question 1

describe the resting potential

Answer 1

neurones are polarised - the inside of the axon is negatively charged (e.g. -70mV) with respect to the outside.

the potential difference across the membrane of an axon when an impulse is not being transmitted is known as the resting potential.

in a polarised neurone there are more sodium ions on the outside of the axon and more potassium ions inside.

although these ions can diffuse through the membrane down their concentration gradients, the resting potential is maintained due to:

1. the membrane being more permeable to the loss of potassium ions than to the intake of sodium ions. this is due to more potassium channels being open than sodium channels resulting in a net loss of positive charged ions from the axon.
2. the sodium/potassium pump actively transports the ions (sodium out, potassium in) against their diffusion gradients. this requires ATP yielded during respiration. this ensures that a concentration gradient is maintained for the movement of ions through the membrane.

Question 2

note…

Answer 2

the sodium-potassium pump functions to maintain the resting potential. sodium and potassium ions diffuse slowly through the membrane.

when neurones are treated with a respiratory inhibitor which prevents production of ATP, the sodium-potassium pump cannot function and the concentration of these ions would eventually reach equilibrium and the potential difference would be zero

Question 3

describe the process of a nerve impulse

Answer 3

when a receptor is stimulated above its threshold it generates an action potential and transmits a nerve impulse. an action potential involves a change in the potential across the axon membrane from a negative inside value e.g. -70mV to a positive inside value e.g. +40 mV. this is known as depolarisation.

depolarisation results from an increase in the permeability of the axon membrane to sodium ions as sodium channel proteins (voltage-dependent gated channels) in the membrane open. the sodium ions diffuse in down a concentration gradient. this causes more sodium channels to open, further increasing the permeability of the membrane to sodium ions (positive feedback). the negative resting potential is initially cancelled out and then as more sodium ions enter a positive charge develops inside the neurons i.e. the axon becomes depolarised.

when the membrane is fully depolarised the sodium channels close (i.e. the permeability of the membrane to sodium ions decreases) and then the potassium channels (voltage-dependent gated channels) in the membrane open so that potassium ions begin to move out of the axon. as potassium ions are positively charged this makes the inside less positive and starts the process of repolarisation.

the sodium/potassium pump restores the resting potential by actively removing sodium ions which have entered and returns potassium ions back into the axon.

Question 4

how is an action potential propagated?

Answer 4

the action potential is propagated (transmitted) along the neurones as a nerve impulse or wave of depolarisation. this is achieved by the increase in permeability of the membrane to sodium ions in one area stimulating an increase in permeability of the next area of the axon and so on along the neurone.

Question 5

what does the refractory period result in and what can the neurone no longer do?

Answer 5

this results in separate discrete impulses

the neurone cannot normally transmit an impulse during this period

Question 6

what is the refractory period?

Answer 6

the time required for the redistribution of the ions that have moved during the action potential in the neurons is known as the refractory period.

the refractory period is one factor involved in determining the maximum frequency of impulses along a neurone.

Question 7

note…

Answer 7

the nature of a nerve impulse is identical irrespective of the type of neurone it travels along.

Question 8

what is the all or nothing principle?

Answer 8

this states that a stimulus must be above a certain ‘threshold level’ or intensity for an impulse to be generated.

any stimulus below this threshold value will not result in the generation of an impulse.

a strong stimulus above the threshold level will result in a greater frequency of impulses than a ‘weak’ stimulus above the threshold.

however, the amplitude (height) of the impulse always remains the same.